The present invention relates generally to a method of fabricating a semiconductor device of improved electrical characteristics which has a deep base portion and more particularly to a method of fabricating a semiconductor device, such as a MOSFET or an insulated gate transistor (IGT), having a deep base portion with a high dopant concentration to provide improved electrical ruggedness.
A method of fabricating semiconductor devices having a deep base portion is described in U.S. Pat. No. 4,443,931, which is assigned to the assignee of the present invention. In accordance with the method described in the patent, a deep P+ base region is formed in a drift region of a power MOSFET prior to formation of the remainder of the base portion. Later processing steps require the growth of an oxide layer on the wafer surface in which the deep P+ base region has been formed. During the subsequent processing steps, this oxide layer has a tendency to leach out some of the dopant of the base region, thus lowering the dopant concentration level and increasing the resistivity of this region in the finished device.
A high dopant concentration in the deep base portion of a MOSFET or of an IGT, both fabricated by similar methods, improves the electrical ruggedness of the device by reducing sporadic disruptive breakdowns of the P-N junction during high voltage operation. Additionally, in an IGT a high dopant concentration in the deep base region decreases the base shunt resistance, thereby reducing parasitic thyristor action of the IGT.
The initial dopant concentration level of the deep P+ base region during device fabrication is limited by several problems, including the permissible density of surface defects caused by doping and the limit of solid solubility of the dopant in a silicon wafer. After the initial doping of the deep P+ base region, subsequent fabrication steps are performed with one or more oxide layers overlying the base region. These oxide layers tend to leach out dopant from the deep P+ base region. Thus, prior art fabrication methods are limited in the impurity concentration levels in the deep base region of the finished device to about 10.sup.18 atoms per cubic centimeter. A further problem in prior art fabrication methods is that the final dopant concentration level of the deep base portion cannot be precisely controlled.